1. Field of the Invention
The present invention pertains to a method and apparatus for increasing the capacity of loads which can be handled by supporting and manipulating equipment. More particularly, the present invention finds use in load handling apparatus including blocks and lines, such as a traveling block to which a load can be connected and a crown block from which the traveling line is suspended by means of a line arrayed between sheaves forming parts of the two blocks. The present invention finds particular application to instances where one of the two blocks is further provided with motion compensating means, such as would be used in a marine environment where wave action would impart movement to the support means.
2. Description of the Prior Art
It is known to use motion compensating devices to compensate for the vertical motion imparted to marine vessels by wave action. The motion compensators are necessary when the vessel is supporting submerged equipment, as for example, during drilling or other undersea operations. Such vessels usually have a derrick fixed to the deck of the vessel with a crown block at the upper end of the derrick and a traveling block suspended from the crown block by a line arrayed between the sheaves of the blocks. The traveling block has a hook from which an elevator or the like is connected to depend into the water to the ocean floor. A fluid actuated system provides a reciprocal motion to allow the hook to be maintained at a fixed location relative to the undersea floor as the traveling block heaves with the wave action imparted to the vessel. Such compensators require the weight of the compensation apparatus to be supported by the same line by which the traveling block is suspended from the crown block which is fixed to the mast or derrick.
Motion compensators are known for keeping either the crown block or the traveling block stationary relative to the undersea floor as the floating vessel moves. In either case, the respective block is supported by means of a pair of direct acting hydraulic or pneumatic piston-cylinder assemblies. Additional compensation apparatus might be required to relieve linear movement of the flexible line supporting the block to prevent relative movement between the two blocks as a result of the vessel movement.
It is desirable and advantageous to provide a compensator supported by fluid and/or gaseous pressure assemblies providing balanced support and positioning to minimize the required height of the derrick or mast supporting the crown block and traveling block combination. It is also desirable to minimize the height of the derrick by providing maximum mechanical advantage so that a relatively large motion of the derrick can be compensated by a lesser relative movement between the compensator assembly components. It is also desirable to have the capability of substantially increasing the load handling capacities of the assembly during certain operations requiring that higher than normal loads be supported by the compensating means without unduly affecting the rest of the apparatus so as to maintain a high degree of motion sensitivity while supporting and compensating normal loads. It would not be cost effective to design the apparatus for such peak load capacities under all situations nor could load sensitivity be maintained if the apparatus were designed for the higher loads.